Looks like a cal/mag deficiency to me, either its not getting any because there isnt any to get or the PH is off

https://www.growweedeasy.com/cannabis-plant-problems/calcium-deficiency

Depending on your environment and if its 3 weeks from the flip to 12/12 and you only have a few pistils showing you can also spray the leaves with some Epsom Salt diluted in water, I use a tsp per gallon. If its 3 weeks from the first sign of flower I wouldn't foliar feed.